Engineers

Engineers are quite possibly the most useful unit in Civilization II. They're an upgrade of Settlers that you can produce after you've discovered the secrets of Explosives. Everything that Settlers can do, Engineers can do better. Plus, they can terraform land and clean up pollution. Settlers are also infinitely useful in combat, as long as you've discovered Railroads and you know how to use them correctly....

The most powerful units in CivII, at least until you develop Armor, have a movement of 1. What that means is that when you're trying to surround a city, one of two things will happen. First, you'll arrive just outside the city at the end of your turn, and then have your unit attacked from within the city walls. Since the best offensive units have the worst defense, you'll probably end up losing the unit. The second option is to follow a road to the city, which leaves your unit at either 2/3 or 1/3 strength. If you attack, you'll probably lose, and if you don't, you'll be attacked again, because you're right outside the city walls.

So here's what to do. Rather than build a bunch of defensive units to protect your Cannons, Artillery, and whatnot, build a single shitty defensive unit (Musketeer, Rifleman), and use him to protect an Engineer or two while you build a railroad from your city to the city you want to attack. Once the railroad is finished, pull back your Engineers and your defensive unit. Now you've got an immediate attack conduit from your city to his -- since units can move along railroads at no penalty, you waste no movement points attacking from your city to your opponents'. Throw in a couple of Cavalry units, and you can attack and retreat in the same round. Or you can use Diplomats and Spies to tear shit down from the inside while you build up your forces. And once you take over that city, you just repeat the process over and over again.